Scraper for a Device for Breaking Bath Crust in an Electrolytic Cell Intended for Aluminium Production

ABSTRACT

The invention relates to a scraper ( 40 ) for a crust breaking member ( 33 ) such as a crust breaker, useable to form an opening in an alumina and solidified bath crust in a fused bath electrolytic cell by a back and forth motion of the said member along a translation axis T. The scraper ( 40 ) according to the invention comprises a plurality of fingers ( 42 ) arranged around a reference axis S intended to coincide with the said translation axis T during use, so as to leave a space ( 44 ) between the fingers and the crust breaking member, and each finger ( 42 ) comprises at least one scraping member ( 43 ) projecting towards the crust breaking member ( 33 ). The scraper according to the invention is used to prevent the formation of an envelope on the surface of the crust-breaking member that can block the member in the scraper.

FIELD OF THE INVENTION

The invention relates to electrolytic cell equipment intended for the production of aluminium by fused bath electrolysis according to the Hall-Heroult process. In particular, it relates to devices for breaking solid bath crusts in the said electrolytic cells.

STATE OF THE ART

Operation of an aluminium production cell by fused bath electrolysis of alumina dissolved in a bath based on cryolite causes a permanent change to the bath composition. Firstly, the alumina is consumed by electrolysis reactions, and secondly the bath quantity and composition are progressively modified by secondary mechanisms such as absorption of cryolite constituents by the walls of the cell or the decomposition of fluoride constituents by anode effects. Consequently, it is important to add alumina and bath compounds such as cryolite (Na₃AIF₆) or aluminium fluoride (AIF₃) regularly to stabilise cell operation parameters.

The alumina and bath compounds are usually added into the bath in the form of a powder. Electrolytic cells are usually equipped with one or several powder distributors associated with a device for breaking the alumina and solidified electrolyte crust that covers the bath surface during normal operation, so that the powder material can be added into the electrolyte bath. The crust breaking device usually comprises a jack and a crust breaking member called a “crust breaker”, “plunger” or “crust breaker chisel” fixed to the end of the jack rod. The crust breaker, usually placed vertically, is lowered by activation of the jack and breaks the alumina and solidified bath crust. The crust breaking devices usually also comprise a scraper to eliminate solidified bath depositions that tend to form on crust breakers.

French patents FR 2 455 093, FR 2 487 386 (corresponding to American patent U.S. Pat. No. 4,431,491) and FR 2 527 647 (corresponding to American patent U.S. Pat. No. 4,437,964) in the name of Aluminium Pechiney describe alumina point feeding devices to an electrolytic cell designed for production of aluminium. These devices, known as “crust breakers—feeders” when they are combined, include alumina supply means and a crust breaking device for breaking the alumina and solidified bath crust. The crust breaking device is provided with a scraper that encircles the crust breaker tightly.

However, the solidified bath sometimes forms an envelope on the surface of the crust-breaking member that can block the member in the scraper. The Applicant attempted to find means of avoiding this disadvantage.

DESCRIPTION OF THE INVENTION

An object of the invention is a scraper for a crust breaking member (such as a crust breaker, a plunger or a crust breaker chisel) useable to form an opening in an alumina and solidified bath crust in a fused bath electrolytic cell by a back and forth motion of the crust breaking member along a translation axis T.

The scraper according to the invention comprises at least three fingers arranged around a reference axis S that will coincide with the said translation axis T during use so as to form a space called a “clearance” between the fingers and the crust breaking member, and each finger comprises at least one scraping member projecting towards the crust breaking member.

The Applicant had the idea of separating the scraper into a set of scraping members separated from each other, reducing the contact area with the scraper and arranging the scraping members on a plurality of fingers at an angle spacing from each other. This approach prevents the crust breaking member from getting blocked by limiting the force necessary to bring it free if there is an accumulation of bath crust between the scraping members and the crust breaking member. The applicant also had the idea of moving the fingers well away from the crust breaking member of the crust breaking device, so as to encourage the accumulation of bath crust debris above the scraping members, rather than their insertion between the jacket and the crust breaking member. The accumulated debris can then easily be evacuated through the space formed by the separation between the scraper fingers.

The invention is particularly suitable for the electrolytic cells to be used for aluminium production.

Another object of the invention is a crust breaking device intended for an aluminium production cell using fused bath electrolysis and comprising a scraper according to the invention.

Another object of the invention is a crust breaker - feeder for use in an aluminium production cell using fused bath electrolysis and comprising a scraper according to the invention.

Another object of the invention is a crust breaking and measurement device intended for use in an aluminium production cell using fused bath electrolysis and comprising a scraper according to the invention.

Another object of the invention is the use of the scraper according to the invention in an electrolytic cell intended for the production of aluminium by fused bath electrolysis.

The invention is described in detail below with reference to the appended figures.

FIG. 1 is a simplified diagram showing a partial inner view of a typical electrolytic cell intended for the production of aluminium by fused bath electrolysis, seen in a vertical section.

FIG. 2 shows a perspective view of a preferred embodiment of the scraper according to the invention.

FIG. 3 shows a side view of the scraper in FIG. 2.

FIGS. 4 and 5 show sectional views of the scraper in FIG. 2, along the sectional planes indicated in FIG. 3.

FIG. 6 shows a perspective view of the scraper in FIG. 2 fixed to the end of a crust breaking device.

As shown in FIG. 1, an electrolytic cell (1) for aluminium production by fused bath electrolysis, in other words by molten salt electrolysis, comprises a pot (2), one or several anodes (3) (typically prebaked anodes made of a carbonaceous material), and feeding means for powder materials (20, 30), usually fixed to a superstructure (4) arranged above the pot.

The electrolytic pot (2) comprises inner lining means (not shown) and a cathode assembly (5) that form a crucible inside the pot (2) that can contain the electrolytic bath and a liquid aluminium pad (6) when the cell is in operation. The anodes (3) are normally partially immersed in the liquid electrolytic bath (7) and the cells are operated so as to form an alumina and solidified bath crust (10) above the electrolytic bath.

The powder material supply means (20, 30) usually comprise a powder material distributor (20) and a crust breaking device (30).

The powder material distributor (20) typically comprises a hopper (21) that will contain a reserve of powder material, and a chute (22) fixed to the lower part of the hopper and intended to carry the powder material close to an opening (11) in the crust (10).

The crust breaking device (30) comprises an actuator (31) and a crust breaking member (33) (often called a “crust breaker”, “plunger” or “crust breaker chisel”) fixed to the end of the actuator rod which is typically located in a jacket or sheath (32). The crust breaking member (33) is usually arranged so as to enter into and exit from the crust breaking device (30) through an opening located at the free end (34) of the jacket or sheath (32). The crust breaking member (33) forms an opening (11) in an alumina and solidified bath crust (10) by a to and fro movement of the crust breaking member (33) along a translation axis T that is typically vertical or slightly inclined from the vertical. The cross-section of the crust breaking member (33) is typically circular. The actuator (31) is typically a pneumatic actuator such as a pneumatic jack. A scraper (40) cleans the crust breaking member during its to and fro movements.

A powder material distributor can be associated with one or several predetermined crust breaking devices, or conversely a crust breaking device may be associated with one or several determined powder material distributors. Electrolytic cells are frequently used with one or several devices comprising a powder materials distributor and a crust breaker device. These devices are known under the term Crustbreaking and Feeding Devices.

During operation, at least one opening (11) is formed (or possibly kept open) in the said crust (10) between the anodes (3), using the crust breaking device(s) (30), and powder material is added into the electrolytic bath (7) through the opening (11) (or through at least one opening when there are several).

The scraper (40) according to the invention comprises a plurality of fingers (42) arranged around a reference axis S that will be coincident with the said translation axis T during use. The fingers (42) are arranged so as to leave a space (44) called a “clearance” between the fingers (42) and the crust breaking member (33). Each finger (42) comprises at least one scraping member (43) that projects towards the crust breaking member (33).

FIGS. 2 to 6 show a preferred embodiment of a scraper according to the invention.

The scraper (40) according to the invention comprises at least three fingers (42) and preferably at least 6 fingers. The number of fingers is usually between 6 and 10 inclusively. The example embodiment of the invention shown in FIGS. 3 to 6 comprises six fingers.

The clearance (44) between the fingers (42) and the crust breaking member (33)—in other words the width E of the space separating the inner surface (421) of the fingers (42) and the outer lateral surface (331) of the crust breaking member (33)—is advantageously more than 10 mm.

The length L1 of the fingers (42) is preferably more than 100 mm.

The fingers (42) are arranged so as to form a space between them, called a separation (422, 423).

The cross section of the fingers (42) is advantageously substantially uniform over at least 50% of their length L1 so as to maintain a spacing (422, 423) over a significant length between the fingers, sufficient to facilitate evacuation of bath crust debris tom off by the scraping members (43). The fingers (42) are preferably straight and parallel to the reference axis S of the scraper.

The distance Cm between the points closest to the surface of the adjacent fingers is preferably greater than 10 mm, and more preferably greater than 20 mm over at least 50% of their length L1, to enable efficient evacuation of bath crust debris produced by scraping. The separation C between adjacent fingers preferably increases with increasing distance from S axis, in other words the distance between the inside and the outside of the scraper, so as to form an opening that flares outwards, and thus to facilitate evacuation of bath crust debris towards the outside of the scraper. In the example shown in FIG. 5, the separation C between the inner end (425) of the fingers and their outer end (426) increases. Preferably, the fingers (42) comprise at least a first plane surface element (424) and a second plane surface element (424′) arranged so that the first surface element (424) of each finger and the second surface element (424′) of an adjacent finger are facing each other and at an angle from each other so as to form an opening that flares towards the outside of the scraper. The said facing surfaces (424, 424′) are preferably separated by an angle α of more than 10° with respect to the said S axis, so as to create a sufficient spacing (422, 423) between the fingers to enable evacuation of bath crust debris produced by scraping.

The angular centre-to-centre spacing β between the fingers is typically between 20° and 90°.

Each scraping member (43) is preferably arranged on the fingers (42) so as to leave a free space (45) called a “scraping space” between the scraping member (43) and the crust breaking member (33). More precisely, each scraping member (43) comprises one end (431) called the “scraping end” located at a determined distance D1 from the said axis S so as to leave the said scraping space. The determined distance D1 is advantageously the same for all scraping ends (431). The scraping space (45) is typically between 1 and 5 mm inclusively.

The scraping distance D1 is less than the clearance (44) (corresponding to a constant separation E in the embodiment shown in FIGS. 3 to 6). The scraping member (43) thus forms a projection facing towards the crust breaking member (33).

Advantageously, each scraping member (43) is located at the free end (420) of each finger (42).

According to one advantageous embodiment of the invention, the scraping end (431) is in the form of a concave surface. The length L2 of this surface along the direction of the S axis is typically between 10 mm and 50 mm and more typically between 10 mm and 30 mm. The area of the concave surface is typically between 10 and 1000 mm². The said concave surface is typically parallel to the outer lateral surface (331) of the crust breaking member (33), in other words it matches the shape of the lateral outer surface (331).

According to another embodiment of the invention, the scraping end (431) is in the form of a tip or an edge.

The fingers (42) are typically fixed to a support element (41). The scraping members (43) are preferably arranged at a determined distance D3 from the support element (41) equal to at least 100 mm.

The scraper (40) is advantageously removable so that it can be changed, particularly in the case of damage or wear. In this case, the support element (41) typically comprises attachment means (410, 411) such as a collar or flange (410) provided with through holes (411). For example, the attachment means (410, 411) are used to fix the scraper to a crust breaking device, typically to the end (34) of a sheath (or jacket) (32) from which the crust breaking member (33) projects. The support element or body (41) and the fingers (42) are typically in a single part.

The fingers (42) are typically made of steel to achieve sufficient stiffness.

FIGS. 4 and 5 show the different parameters for a scraper according to a preferred embodiment of the invention, in which the fingers are straight and have a substantially uniform cross section over more than 50% of their length L1, and for a crust breaking member (33) with a circular section. In these figures, R is the radius of the crust breaking member (33), D1 is the said determined distance, D2 is the distance between the S axis and the central axis of the fingers (42) and E is the said clearance.

The scraper (40) according to the invention may advantageously be used on crust breaking devices (30) used in aluminium production cells by fused bath electrolysis with a crust breaking member free to move in translation along a T axis. In particular, it can advantageously be used on crust-breakers—feeders, like those described above, for use in aluminium production cells by fused bath electrolysis. The scraper (40) according to the invention can be fitted on crust breaking and measurement devices like that described in patent EP 0 716 165 (corresponding to American patent U.S. Pat. No. 6,065,867) in the name of Aluminium Pechiney. In these devices, the crust breaking member also comprises measurement means such as a thermocouple, or can be used as a measurement means (for example for use as an electric contact tip to detect when the member comes into contact with the liquid electrolytic bath during its vertical movements). 

1. A scraper for a crust breaking member in a crust breaking device useable to form an opening in an alumina and solidified bath crust in a fused bath electrolytic cell by a back and forth motion of the crust breaking member along a translation axis, the scraper comprising at least three fingers arranged around a reference axis that coincides with the translation axis during use, so as to form a space defined as a “clearance” between the fingers and the crust breaking member, and wherein each finger comprises at least one scraping member projecting towards the crust breaking member.
 2. The scraper of claim 1, wherein the number of fingers is between 6 and 10 inclusively.
 3. The scraper of claim 1, wherein the length of the fingers is greater than 100 mm.
 4. The scraper of claim 1, wherein the cross section of each finger is substantially uniform over at least 50% of its length.
 5. The scraper of claim 1, wherein each finger is straight and parallel to the reference axis of the scraper.
 6. The scraper of claim 1, wherein the distance between the points closest to the surface of the fingers adjacent to each other is greater than 10 mm over at least 50% of their length.
 7. The scraper of claim 1, wherein the separation between adjacent fingers increases with increasing distance from the reference axis so as to form an opening that flares outwards.
 8. The scraper of claim 7, wherein each finger comprises at least a first plane surface element and a second plane surface element arranged so that the first surface element of each finger and the second surface element of an adjacent finger are facing each other and at an angle from each other so as to form an opening that flares towards the outside of the scraper.
 9. The scraper of claim 8, wherein the angle is more than 10° with respect to the reference axis.
 10. The scraper of claim 1, wherein the clearance between each finger and the crust breaking member is more than 10 mm.
 11. The scraper of claim 1, wherein each finger has a free end and wherein the at least one scraping member is located at the free end of the finger.
 12. The scraper of claim 1, wherein each finger is fixed to a support element.
 13. The scraper of claim 12, wherein the at least one scraping members is arranged at a determined distance from the support element equal to at least 100 mm.
 14. The scraper of claim 13, wherein the support element comprises attachment means.
 15. A crust breaking device intended for an aluminium production cell using fused bath electrolysis, the crust breaking device comprising: a scraper for a crust breaking member in the crust breaking device useable to form an opening in an alumina and solidified bath crust in the fused bath electrolytic cell by a back and forth motion of the crust breaking member along a translation axis, the scraper comprising at least three fingers arranged around a reference axis that coincides with the translation axis during use, so as to form a space defined as a “clearance” between the fingers and the crust breaking member, and wherein each finger comprises at least one scraping member projecting towards the crust breaking member.
 16. A crust-breaker—feeder for use in an aluminium production cell using fused bath electrolysis and comprising a scraper for a crust breaking member in a crust breaking device useable to form an opening in an alumina and solidified bath crust in a fused bath electrolytic cell by a back and forth motion of the crust breaking member along a translation axis, the scraper comprising at least three fingers arranged around a reference axis that coincides with the translation axis during use, so as to form a space defined as a “clearance” between the fingers and the crust breaking member, and wherein each finger comprises at least one scraping member projecting towards the crust breaking member.
 17. A crust breaking and measurement device adapted for use in an aluminium production cell using fused bath electrolysis and comprising a scraper for a crust breaking member in the crust breaking and measurement device useable to form an opening in an alumina and solidified bath crust in a fused bath electrolytic cell by a back and forth motion of the crust breaking member along a translation axis, the scraper comprising at least three fingers arranged around a reference axis that coincides with the translation axis during use, so as to form a space defined as a “clearance” between the fingers and the crust breaking member, and wherein each finger comprises at least one scraping member projecting towards the crust breaking member.
 18. Use of the scraper of claim 1, in an aluminium production cell using fused bath electrolysis. 